P Huber

Viral haemorrhagic septicaemia virus IVb experimental infection of rainbow trout, Oncorhynchus mykiss (Walbaum), and fathead minnow, Pimphales promelas (Rafinesque)

Viral haemorrhagic septicaemia virus (VHSV) in the Great Lakes has had a dramatic impact on fish husbandry because of the implications of the presence of a reportable disease. Experimental infections with VHSV IVb were conducted in rainbow trout, Oncorhynchus mykiss (Walbaum), and fathead minnows, Pimphales promelas (Rafinesque), to examine their susceptibility and the clinical impact of infection. Triplicate groups of rainbow trout (n = 40) were injected intraperitoneally (i.p.) with 100 microL 10(6.5)50% tissue culture infective doses (TCID(50)) or waterborne exposed to graded doses (10(4.5), 10(6.5), and 10(8.5) TCID(50) mL(-1)) of VHSV IVb. Duplicate groups of fathead minnows (n = 15) were i.p. injected with (10(6.5) TCID(50) 100 microL) or waterborne exposed (10(6.5) TCID(50) mL(-1)). All experiments were performed with single-pass well water maintained at 12 degrees C. Following either i.p. or waterborne exposure, VHSV RNA was detectable in both rainbow trout and fathead minnows by nested reverse transcription polymerase chain reaction (nRT-PCR) as early as 4-7 days post-infection (p.i.). Infected fathead minnow and rainbow trout exhibited lesions characteristic of VHS at 9 and 15 days p.i., respectively. Route of exposure had little effect on the onset of clinical signs. Cumulative mean mortality in rainbow trout was 4.4%, 2.6%, 2.6% and less than 1% in the i.p., high, medium and low dose waterborne exposures, respectively. Cumulative average mortality of 50% and 13% occurred in i.p. and waterborne-exposed fathead minnows, respectively. VHSV was detected from pooled rainbow trout tissue by RT-PCR and virus isolation at 38 days p.i., but not at 74 days p.i., regardless of the exposure route. Immunohistochemistry (IHC) with a rabbit antibody to VHSV IVb revealed the viral tissue tropisms following infection, with the identification of viral antigen in myocardium and necrotic branchial epithelium of both species and in gonadal tissue of fathead minnows. Rainbow trout, but not fathead minnows, are relatively refractory to experimental infection with VHSV IVb.

An immune-complex glomerulonephritis of Chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Chinook salmon from New Zealand were shown to have a generalized membranous glomerulonephritis that was most severe in large fish. Marked thickening of the glomerular basement membrane was the most consistent lesion, with the presence of an electron-dense deposit beneath the capillary endothelium.Severely affected glomeruli also had expansion of the mesangium and loss of capillaries,synechiae of the visceral and parietal epithelium and mild fibrosis of Bowmans capsule. Chinook salmon from British Columbia, Canada with bacterial kidney disease caused by Renibacterium salmoninarum had similar histological lesions. They also had thickened glomerular basement membranes that were recognized by rabbit antiserum to rainbow trout immunoglobulin. This was true only when frozen sections of kidney were used and not formalin-fixed tissue. An attempt to experimentally produce a glomerulopathy in rainbow trout by repeated immunization with killed R. salmoninarum was not successful. Case records from the Fish Pathology Laboratory at the University of Guelph over a 10-year period revealed that a range of species were diagnosed with glomerulopathies similar to those seen in Chinook salmon. The majority of these cases were determined to have chronic inflammatory disease. This report has identified the presence of immunoglobulin within thickened basement membranes of Chinook salmon with glomerulonephritis and supports the existence of type III hypersensitivity in fish.

Plasma ladderlectin concentration following sterile inflammation and Aeromonas salmonicida subsp. salmonicida infection

Plasma samples obtained from rainbow trout either experimentally infected with Aeromonas salmonicida or injected with either A. salmonicida lipopolysaccharide (LPS) or a commercial A. salmonicida vaccine (Lipogen) were analysed by enzyme immunoassay to evaluate changes in rainbow trout ladderlectin (RTLL) concentrations during the acute phase response (APR). Plasma RTLL concentrations in fish injected with A. salmonicida LPS, vaccine or live A. salmonicida varied over a 10 day period, but did not significantly increase. In contrast, fish experimentally infected with A. salmonicida exhibited a modest, but statistically significant (P < 0.05), decrease in RTLL concentration. These studies demonstrate that RTLL is not detectably induced during the trout APR to sterile inflammation or A. salmonicida infection, but plasma concentration of this protein may be reduced during bacterial infection.

Dermocystidium-like organism linked with a mortality event in yellow perch Perca flavescens (Mitchill) in Ontario, Canada.

Organisms classified as Dermocystidium spp. are Mesomycetozoea within the order of Dermocystida and are uniquely positioned at the boundary of animals and fungi (Gozlan et al. 2014). There are at least 20 known species of Dermocystidium (Pekkarinen & Lotman 2003), but this is considered an underestimation as most have only been identified to the genus level (Gozlan et al. 2014). Dermocystidium are found worldwide and may live in brackish water, or oligotrophic to slightly eutrophic lakes (Pekkarinen & Lotman 2003). They are known pathogens of a variety of freshwater fish species and amphibians (Pascolini et al. 2003). The outcome of infection ranges from subclinical disease and low-level chronic mortalities to rapidly fatal outbreaks. Common perch, Perca fluviatilis Linnaeus, in Finland (Pekkarinen & Lotman 2003), Germany (Reichenbach-Klinke 1950 via Pekkarinen & Lotman 2003), Estonia (Pekkarinen & Lotman 2003) and the United Kingdom (UK) (Morley, Campbell & Lewis 2008) are known to be infected by three different species of Dermocystidium; D. percae, D. fennicum and D. sp. These species differ in tissue distribution and macroand microscopic appearance of the cyst. Highlighting the pleomorphism of this genus, Dermocystidium koi, a species that infects Cyprinus carpio Linnaeus var. koi has an additional hyphal form (Dykova & Lom 1992). The variable tissue tropisms are theorized to be a product of microhabitat competition as different species of Dermocystidium have the ability to co-infect a single fish (Morley et al. 2008). On a 60-mile stretch of the Wye River in Ontario, Canada (44 43 55N, 7950 47W), there was a rapid temperature decline to freezing and die off of approximately 200 yellow perch, Perca flavescens (Mitchill). The surviving yellow perch were observed normally schooling off the banks, and no other fish species were affected. In early December 2009, six moribund fish were preserved in 10% buffered formalin and submitted to the University of Guelph for post-mortem. On gross examination, the moribund fish were small for young of the year at between 6 and 6.5 cm length, and had a myriad of single 1 mm, round to coalescing, firm-raised white nodules or cysts randomly distributed across the skin, fins and corneas (Fig. 1). No other gross lesions were found, but the gills had undergone autolysis and, hence, were non-diagnostic. Though embedded in the dermis, the cysts were easily detached. A wet mount of the cysts acquired by skin scrapping revealed multiple 130 9 200 lm round cysts packed with hundreds of endospores (ranging from 5 to 7 9 6 to 12 lm) with a central Correspondence J S Lumsden, Fish Pathology Laboratory, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada (e-mail: jsl@uoguelph.ca)